Selecting carriers for modulating signals in a communication network

ABSTRACT

Communicating over a network of communication stations includes monitoring for reception of an electromagnetic wave that has a power above a threshold and a frequency in a predetermined frequency interval, and selecting carrier frequencies for modulating information onto signals transmitted over the network. The carrier frequencies are selected based at least in part on whether an electromagnetic wave having a power above the threshold and a frequency in the frequency interval has been received in a predetermined time interval, and based at least in part on a demand by one or more stations in the network for capacity for communicating over the network.

TECHNICAL FIELD

The invention relates to selecting carriers for modulating signals in acommunication network.

BACKGROUND

Various types of communication systems transmit signals that may radiatein a portion of the electromagnetic spectrum and cause interference withdevices that operate in that portion of the electromagnetic spectrum(e.g., radio frequency (RF) spectral bands). In some cases regulatoryrequirements for certain geographical regions (e.g., imposed bygovernments) place constraints on power that may be radiated in certainspectral regions, such as amateur radio bands. Some systems are wirelesssystems that communicate between stations using radio waves modulatedwith information. Other systems are wired systems that communicate usingsignals transmitted over a wired medium, but the wired medium mayradiate enough power in restricted spectral bands to potentially causeinterference.

Communication stations can be configured to avoid using or limit theamount of power that is radiated in certain restricted spectral bands.Alternatively, communication stations can be configured to adjust thespectral regions used for communication, based on whether the station isoperating in an environment in which interference may occur. Forexample, orthogonal frequency division multiplexing (OFDM), also knownas Discrete Multi Tone (DMT), is a spread spectrum signal modulationtechnique in which the available bandwidth is subdivided into a numberof narrowband, low data rate channels or “carriers.” To obtain highspectral efficiency, the spectra of the carriers are overlapping andorthogonal to each other. Data are transmitted in the form of symbolsthat have a predetermined duration and encompass some number ofcarriers. The data transmitted on these carriers can be modulated inamplitude and/or phase, using modulation schemes such as Binary PhaseShift Key (BPSK), Quadrature Phase Shift Key (QPSK), or m-bit QuadratureAmplitude Modulation (m-QAM).

SUMMARY

In one aspect, in general, the invention features a method forcommunicating over a network of communication stations. The methodincludes monitoring for reception of an electromagnetic wave that has apower above a threshold and a frequency in a predetermined frequencyinterval. The method also includes selecting carrier frequencies formodulating information onto signals transmitted over the network. Thecarrier frequencies are selected based at least in part on whether anelectromagnetic wave having a power above the threshold and a frequencyin the frequency interval has been received in a predetermined timeinterval, and based at least in part on a demand by one or more stationsin the network for capacity for communicating over the network.

In another aspect, in general, the invention features an apparatus. Theapparatus includes a monitoring module including circuitry formonitoring for reception of an electromagnetic wave that has a powerabove a threshold and a frequency in a predetermined frequency interval.The apparatus includes a modulation module including circuitry formodulating information onto signals transmitted over a network. Theapparatus includes a carrier selection module including circuitry toselect carrier frequencies for use by the modulation module. The carrierfrequencies are selected based at least in part on whether anelectromagnetic wave having a power above the threshold and a frequencyin the frequency interval has been received in a predetermined timeinterval, and based at least in part on a demand by one or more stationsin the network for capacity for communicating over the network.

Aspects of the invention may include one or more of the followingfeatures.

The demand for capacity by a station comprises a request fortransmission of data from the station at a rate that exceeds anavailable capacity over a communication medium shared by multiplestations in the network.

The method further includes ensuring that carrier frequencies in thefrequency interval are not selected for modulating information ontosignals transmitted over the network if an electromagnetic wave having apower above the threshold and a frequency in the frequency interval hasbeen received in the predetermined time interval.

The method further includes ensuring that carrier frequencies in thefrequency interval that are not selected for modulating information ontosignals transmitted over the network are not used to transmit more thana tenth of the power transmitted using selected carrier frequencies.

The monitoring comprises monitoring for reception of one or morequalified electromagnetic waves that have a power above a threshold anda frequency in one of multiple predetermined frequency intervals in thepredetermined time interval.

A given carrier frequency in a subset of carrier frequencies is selectedfor modulating information onto signals transmitted over the network ifno qualified electromagnetic wave is received in a correspondingfrequency interval that includes the given carrier frequency and if atleast one station demands capacity for communicating over the network.

The method further includes terminating use of the given carrierfrequency for modulating information onto signals transmitted over thenetwork if a qualified electromagnetic wave is received in acorresponding frequency interval that includes the given carrierfrequency or if remaining demand for capacity can be satisfied withoutusing the given carrier frequency.

The method further includes ensuring that the carrier frequencies in thesubset that are not selected for modulating information onto signalstransmitted over the network are not used to transmit more than a tenthof the power transmitted using selected carrier frequencies.

The method further includes transmitting from a first station in thenetwork to at least one other station in the network informationcharacterizing the demand for capacity.

The information characterizing the demand for capacity comprisesinformation characterizing a demand for capacity by the first station.

The monitoring comprises monitoring for reception of one or morequalified electromagnetic waves that have a power above a threshold anda frequency in one of multiple predetermined frequency intervals in thepredetermined time interval.

A given carrier frequency in a subset of carrier frequencies is selectedfor modulating information onto signals transmitted over the network ifno qualified electromagnetic wave is received in a correspondingfrequency interval that includes the given carrier frequency.

The method further includes terminating use of the given carrierfrequency for modulating information onto signals transmitted over thenetwork if a qualified electromagnetic wave is received in acorresponding frequency interval that includes the given carrierfrequency.

The selected carrier frequencies in the subset are used for modulating afirst category of information onto signals transmitted over the networkand are not used for modulating a second category of information ontosignals transmitted over the network.

The first category comprises information that is unicast to one otherstation.

The first category comprises information that is modulated onto a signalusing a modulation format that is adapted to a communication channelbetween two stations.

The second category comprises information that is broadcast to allstations in the network.

The second category comprises preamble or overhead information modulatedonto signals transmitted over the network.

The selected carrier frequencies in the subset are used to modulateinformation according to a predetermined encoding that does not dependon which carrier frequencies in the subset are selected for modulatinginformation.

The predetermined encoding includes interleaving data in a manner thatdoes not depend on which carrier frequencies in the subset are selectedfor modulating information.

At least some carrier frequencies not in the subset of carrierfrequencies are used to modulate information according to apredetermined encoding that does not depend on which carrier frequenciesin the subset are selected for modulating information.

The predetermined encoding includes interleaving data in a manner thatdoes not depend on which carrier frequencies in the subset are selectedfor modulating information.

The carrier frequencies not in the subset are used for modulatinginformation onto signals transmitted over the network regardless ofwhether any qualified electromagnetic waves is received in any frequencyintervals.

The selected carrier frequencies in the subset are used for modulatingredundant information that corresponds to the information modulated onthe carrier frequencies not in the subset.

The selected carrier frequencies in the subset are used for modulatingthe redundant information if the information modulated on the carrierfrequencies not in the subset is being broadcast to multiple stations inthe network.

The selected carrier frequencies in the subset are used for modulatinginformation onto signals sent by a subset of the stations.

The subset of the stations comprises stations that transmit the largestamount of data over the network.

The subset of the stations comprises stations that transmit data overthe network for the largest amount of time.

The subset of the stations comprises stations that use the largestamount of a capacity over a communication medium shared by multiplestations in the network.

The method further includes transmitting from first a station in thenetwork to at least one other station in the network informationindicating whether an electromagnetic wave having a power above thethreshold and a frequency in the frequency interval has been received.

The information indicates whether the electromagnetic wave has beenreceived at the first station.

Each station in the network monitors for reception of an electromagneticwave that has a power above a threshold and a frequency in apredetermined frequency interval.

Selecting the carrier frequencies is performed in a distributed processby multiple of the stations in the network.

A first station selects the carrier frequencies and broadcastsinformation specifying the selected carrier frequencies to otherstations in the network.

The first station selects the carrier frequencies based on whether theelectromagnetic wave has been received at the first station.

The first station selects the carrier frequencies based on whether theelectromagnetic wave has been received at a station other than the firststation.

Transmitting signals over the network comprises transmitting signalsover a wired communication medium.

The wired communication medium comprises a powerline network.

The electromagnetic wave comprises a radio wave.

The predetermined frequency interval comprises an amateur radio band.

The radio wave has a frequency within a band regulated by a governmentagency.

Among the many advantages of the invention (some of which may beachieved only in some of its various aspects and implementations) arethe following.

The adaptive carrier selection technique can be used to provideadditional carriers to increase communication capacity in response to ademand for such capacity when the additional carriers are not likely tointerfere with a licensed operator (e.g., an amateur radio operator).Since there are a limited number of amateur operators, only some ofwhich may actually use the frequencies of the additional carriers, theadditional carriers can often be used without causing interference.These additional “reserved carriers” may be used under certainconditions. By encoding certain communication symbols such as preamble,frame control and robust mode symbols without using the reservedcarriers, the system maintains interoperability among stationsregardless of the status of the reserved carriers.

Other features and advantages of the invention will be found in thedetailed description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a network configuration.

FIG. 2 is a block diagram of a communication system.

DETAILED DESCRIPTION

There are a great many possible implementations of the invention, toomany to describe herein. Some possible implementations that arepresently preferred are described below. It cannot be emphasized toostrongly, however, that these are descriptions of implementations of theinvention, and not descriptions of the invention, which is not limitedto the detailed implementations described in this section but isdescribed in broader terms in the claims.

As shown in FIG. 1, a network 100 provides a shared communication medium110 for a number of communication stations 102A-102E (e.g., computingdevices, or audiovisual devices) to communicate with each other. Thecommunication medium 110 can include one or more types of physicalcommunication media such as coaxial cable, unshielded twisted pair,power lines, or wireless channels for example. The network 100 can alsoinclude devices such as bridges or repeaters. The communication stations102A-102E communicate with each other using predetermined physical (PHY)layer and medium access control (MAC) layer communication protocols usedby network interface modules 106. The MAC layer is a sub-layer of thedata link layer and provides an interface to the PHY layer, according tothe Open Systems Interconnection (OSI) network architecture standard,for example. The network 100 can have any of a variety of networktopologies (e.g., bus, tree, star, mesh).

In some implementations, the network interface modules 106 use protocolsthat include features to improve performance when the network 100includes a communication medium 110 that exhibits varying transmissioncharacteristics. For example, the communication medium 110 may includeAC power lines in a house, optionally coupled to other media (e.g.,coaxial cable lines).

Power-line communication systems use existing AC wiring to exchangeinformation. Owing to their being designed for much lower frequencytransmissions, AC wiring provides varying channel characteristics at thehigher frequencies used for data transmission (e.g., depending on thewiring used and the actual layout). To increase the data rate betweenvarious links, stations adjust their transmission parametersdynamically. This process is called channel adaptation. Channeladaptation results in adaptation information specifying a set oftransmission parameters that can be used on each link. Adaptationinformation includes such parameters as the frequencies used, theirmodulation, and the forward error correction (FEC) used.

The communication channel between any two stations provided by thecommunication medium 110 may exhibit varying channel characteristicssuch as periodic variation in noise characteristics and frequencyresponse. To improve performance and QoS stability in the presence ofvarying channel characteristics, the stations can synchronize channeladaptation with the frequency of the AC line (e.g., 50 or 60 Hz). Thereare typically variations in the phase and frequency of the AC line cyclefrom the power generating plant and local noise and load changes. Thissynchronization enables the stations to use consistent channeladaptation optimized for a particular phase region of the AC line cycle.An example of such synchronization is described in U.S. patentapplication Ser. No. 11/337,946, incorporated herein by reference.

Another aspect of mitigating potential impairments caused by the varyingchannel characteristics involves using a robust signal modulation formatsuch as OFDM. An exemplary communication system that uses OFDMmodulation is described below.

Any of a variety of communication system architectures can be used toimplement the portion of the network interface module 106 that convertsdata to and from a signal waveform that is transmitted over thecommunication medium. An application running on a station provides andreceives data to and from the network interface module 106 in segments.A “MAC Protocol Data Unit” (MPDU) is a segment of information includingoverhead and payload fields that the MAC layer has asked the PHY layerto transport. An MPDU can have any of a variety of formats based on thetype of data being transmitted. A “PHY Protocol Data Unit (PPDU)” refersto the modulated signal waveform representing an MPDU that istransmitted over the power line.

In OFDM modulation, data are transmitted in the form of OFDM “symbols.”Each symbol has a predetermined time duration or symbol time T_(s). Eachsymbol is generated from a superposition of N sinusoidal carrierwaveforms that are orthogonal to each other and form the OFDM carriers.Each carrier has a peak frequency f_(i) and a phase Φ_(i) measured fromthe beginning of the symbol. For each of these mutually orthogonalcarriers, a whole number of periods of the sinusoidal waveform iscontained within the symbol time T_(s). Equivalently, each carrierfrequency is an integral multiple of a frequency interval Δf=1/T_(s).The phases Φ_(i) and amplitudes A_(i) of the carrier waveforms can beindependently selected (according to an appropriate modulation scheme)without affecting the orthogonality of the resulting modulatedwaveforms. The carriers occupy a frequency range between frequencies f₁and f_(N) referred to as the OFDM bandwidth. The stations use anadaptive carrier selection technique, described in more detail below, toselect carriers for transmitting data based on monitoringelectromagnetic spectral regions and monitoring demand for communicationcapacity.

Referring to FIG. 2, a communication system 200 includes a transmitter202 for transmitting a signal (e.g., a sequence of OFDM symbols) over acommunication medium 204 to a receiver 206. The transmitter 202 andreceiver 206 can both be incorporated into a network interface module106 at each station. The communication medium 204 represents a path fromone station to another over the communication medium 110 of the network100. A wireless system 200 includes an antenna for transmitting andreceiving signals over a free space path between stations. The antennacan also be used to enable a monitoring module 221 to monitor forreceived electromagnetic waves. A wired system 200 (e.g., thatcommunicates over power lines) may not need an antenna for transmittingsignals, but can include an antenna 201 that enables a monitoring module221 to monitor for received electromagnetic waves.

At the transmitter 202, modules implementing the PHY layer receive anMPDU from the MAC layer. The MPDU is sent to an encoder module 220 toperform processing such as scrambling, error correction coding andinterleaving. The encoder module 220 can provide redundancy to enableeach portion of an MPDU to be recovered from fewer than all of themodulated carriers or fewer than all of the modulated symbols.

The encoded data is fed into a mapping module 222 that takes groups ofdata bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits), depending on theconstellation used for the current symbol (e.g., a BPSK, QPSK, 8-QAM,16-QAM constellation), and maps the data value represented by those bitsonto the corresponding amplitudes of in-phase (I) and quadrature-phase(Q) components of a carrier waveform of the current symbol. This resultsin each data value being associated with a corresponding complex numberC_(i)=A_(i) exp(jΦ_(i)) whose real part corresponds to the I componentand whose imaginary part corresponds to the Q component of a carrierwith peak frequency f_(i). Alternatively, any appropriate modulationscheme that associates data values to modulated carrier waveforms can beused.

A carrier selection module 223 adaptively selects carrier frequenciesf₁, . . . , f_(N) (or “tones”) within the OFDM bandwidth that are to beused by the system 200 to transmit information according to a “tonemask.” A default tone mask excludes carrier frequencies that are likelyto interfere with licensed entities in a particular region (e.g., NorthAmerica). Devices sold in a given region can be programmed to use adefault tone mask configured for that region.

The carrier selection module 223 can turn on any of a set of “reservedcarriers” when there is no indication that a licensed entity isoperating within a frequency interval that includes the reserved carrierfrequency in the vicinity of the station. An “extended tone mask” isformed by turning on one or more of the reserved carriers. The carrierselection module 223 can send to the encoder module 220 and to themapping module 222 any number of extended tone masks to be used atdifferent times or for different symbols within an MPDU. The encodermodule 220 can use the extended tone mask to determine how data is to beinterleaved. The mapping module 222 can use the extended tone mask todetermine how data values are to be mapped to carrier waveforms.

The carrier selection module 223 receives input signals that are used todetermine whether a given reserved carrier is to be used. One inputsignal indicates whether it is likely that a given reserved carrier canbe used without causing interference to a licensed entity operating inthe vicinity of the station. To determine this likelihood, themonitoring module 221 monitors reception of any electromagnetic wavesthat may be transmitted from such an entity within one or more “reservedfrequency intervals” that include the reserved carrier frequencies. Forexample, the reserved frequency intervals can be narrowband intervalssurrounding each reserved carrier, or reserved frequency intervals caninclude multiple reserved carriers that are turned on or off in sets.The monitoring module 221 generates a signal S_(i) for each monitoredreserved frequency interval that indicates whether a “qualifiedelectromagnetic wave” with a power above a predetermined threshold and afrequency in the reserved frequency interval has been received in thepast T hours. In some cases, the time window T is a short time intervalsuch as 0.5 hour, 1 hour, 2 hours, etc. In some cases, if it is notnecessary to have a fast response to preventing potential interference,the time window T can be a longer time interval such as one or more daysor weeks. The time window T can be different for different reservedfrequency intervals.

Another input signal S_(D) characterizes a demand for communicationcapacity from one or more stations in the network. Each station canrepeatedly broadcast information about whether the station has a requestfor transmission of data from the station at a rate that exceeds acurrently available capacity over the shared communication medium 110using the current tone mask(s). For example, a station may have arequest to transmit or receive a video stream that would consume morecapacity than is available. Each station can transmit a logical signalS_(d) that is true if that station has a demand for communicationcapacity. The signal S_(D) can then be calculated as the logical ORoperation performed on the individual S_(d) signals.

When S_(i) indicates that no qualified electromagnetic wave has beenreceived for a given reserved frequency interval and S_(D) indicatesthat there is a demand for more capacity, the one or more carrierfrequencies within the reserved frequency interval are added to anextended tone mask. In networks that have a central controller (CCo)station that coordinates communication on the network, the CCo stationcan make this determination and broadcast the new extended tone mask tothe other stations in the network. Alternatively, the CCo station maybroadcast the new extended tone mask to only those stations occupyingsignificant portions of time on the shared communication medium 110(e.g., to limit the use of the reserved carriers to those stations thatwould benefit most from the extra capacity provided by the reservedcarriers). If the extended tone mask is sent to stations, those stationsdo not necessarily need to have a carrier selection module 223.Alternatively, if each station does have a carrier selection module 223,the CCo station can broadcast the S_(i) and S_(D) input signals and eachstation can determine the extended tone mask based on these signals fromthe CCo station. In some networks, multiple stations in the network canmonitor reception of qualified electromagnetic waves in each reservedfrequency interval and send the monitoring results to the CCo station toincrease the reliability of detecting the waves (e.g., a wave may have alow power just above the threshold which can be detected by a station inone portion of the network, but not by the CCo station).

Other protocols for determining an extended tone mask can be used thatdo not necessarily require a CCo station. For example, in networkswithout a CCo station (e.g., a CSMA network) stations can occasionallybroadcast information characterizing reception of qualifiedelectromagnetic waves and demand for capacity so that the decision aboutwhether to include a given reserved carrier in the extended tone mask ismade using a distributed algorithm (e.g., distributed consensusalgorithms). Additionally, a pair of stations in the network 100 maychoose to use reserved carriers in an extended tone mask based on theirreception (or lack thereof) of one or more qualified electromagneticwaves and their demand for capacity, independently of the otherstations. It is not necessary that the stations inform other stations ofthe extended tone mask, but there may be cases where communicating theextended tone mask to other stations may be useful. In someimplementations, in a network 100 that includes a CCo station, the CCocan allow the option for individual pairs of communicating stations toselect an extended tone mask for use between that pair of stations.

When the additional network capacity provided by one or more reservedcarriers is no longer needed a new tone mask can be generated withoutthose carriers. Also, if a qualified electromagnetic wave is receivedwith a frequency in a reserved frequency interval while correspondingreserved carriers in that interval are being used in an extended tonemask, a new tone mask can be generated without the correspondingreserved carriers.

To increase reliability and interoperability, certain communicationsymbols such as symbols carrying overhead information (e.g., preamblesymbols and frame control symbols), and symbols carrying informationbroadcast to all stations (e.g., robust mode symbols) can be encoded andmodulated using the default tone mask without any reserved carriersregardless of whether or not any other station is using an extended tonemask. Alternatively, certain communication symbols can be encoded andmodulated to make use of the reserved carriers only for communicatingredundant information that is not required for correctly demodulatingand decoding the symbols.

One technique that facilitates interoperability for certaincommunication symbols includes using a predetermined encoding that doesnot depend on which reserved carrier frequencies are selected forinclusion in the extended tone mask. For example, the encoder module 220interleaves data in a manner that does not depend on which reservedcarrier frequencies are used. The interleaving can include all reservedcarriers regardless of whether the carrier is selected, and can ensurethat only redundant information (e.g., copies or parity bits) is mappedto the reserved carriers. Thus, a station can demodulate and decodeinformation from a given default carrier frequency or a given reservedcarrier frequency without necessarily knowing which reserved carriersmay have been selected for modulating information in the extended tonemask. A station can demodulate and decode information without using anyreserved carriers, or a station can use one or more reserved carriers toobtain redundant information that may be used, for example, to correcterrors.

The mapping module 222 also determines the type of modulation to be usedon each of the carriers in the tone mask according to a “tone map.” Thetone map can be a default tone map (e.g., for redundant broadcastcommunication among multiple stations), or a customized tone mapdetermined by a receiving station that has been adapted tocharacteristics of the communication medium 204 (e.g., for moreefficient unicast communication between two stations). If a stationdetermines (e.g., during channel adaptation) that a carrier in the tonemask is not suitable for use (e.g., due to fading or noise) the tone mapcan specify that the carrier is not to be used to modulate data, butinstead can use pseudorandom noise for that carrier (e.g., coherent BPSKmodulated with a binary value from a Pseudo Noise (PN) sequence). Fortwo stations to communicate, the transmitted symbols should use the sametone mask and tone map, or the stations should at least know what tonemask and tone map the other station is using so that the symbols can bedemodulated properly. In some networks, a pair of stations that performschannel adaptation do determine a tone map may also decide to turn onone or more reserved carriers in a reserved frequency interval in whichno qualified electromagnetic wave has been detected to establish anextended tone mask to be used just for unicast communication between thetwo stations, as described above. Other stations using a differentextended tone mask or the default tone mask would not need to know thatextended tone mask established by those stations.

A modulation module 224 performs the modulation of the resulting set ofN complex numbers (some of which may be zero for unused carriers)determined by the mapping module 222 onto N orthogonal carrier waveformshaving peak frequencies f₁, . . . , f_(N). The modulation module 224performs an inverse discrete Fourier transform (IDFT) to form a discretetime symbol waveform S(n) (for a sampling rate f_(R)), which can bewritten as

$\begin{matrix}{{S(n)} = {\sum\limits_{i = 1}^{N}\;{A_{i}{\exp\left\lbrack {j\left( {{2{{\pi{\mathbb{i}n}}/N}} + \Phi_{i}} \right)} \right\rbrack}}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where the time index n goes from 1 to N, A_(i) is the amplitude andΦ_(i) is the phase of the carrier with peak frequency f_(i)=(i/N)f_(R),and j =√−1. In some implementations, the discrete Fourier transformcorresponds to a fast Fourier transform (FFT) in which N is a power of2.

A post-processing module 226 combines a sequence of consecutive(potentially overlapping) symbols into a “symbol set” that can betransmitted as a continuous block over the communication medium 204. Thepost-processing module 226 prepends a preamble to the symbol set thatcan be used for automatic gain control (AGC) and symbol timingsynchronization. To mitigate intersymbol and intercarrier interference(e.g., due to imperfections in the system 200 and/or the communicationmedium 204) the post-processing module 226 can extend each symbol with acyclic prefix that is a copy of the last part of the symbol. Thepost-processing module 226 can also perform other functions such asapplying a pulse shaping window to subsets of symbols within the symbolset (e.g., using a raised cosine window or other type of pulse shapingwindow) and overlapping the symbol subsets.

An Analog Front End (AFE) module 228 couples an analog signal containinga continuous-time (e.g., low-pass filtered) version of the symbol set tothe communication medium 204. For example, in a wired system thecommunication medium 204 supports propagation of an electrical signalover a wire between stations, and in a wireless system the communicationmedium 204 supports propagation of electromagnetic waves through pathsbetween antennas of stations. The effect of the transmission of thecontinuous-time version of the waveform S(t) over the communicationmedium 204 can be represented by convolution with a function g(τ;t)representing an impulse response of transmission over the communicationmedium. The communication medium 204 may add noise n(t), which may berandom noise and/or narrowband noise emitted by a jammer.

At the receiver 206, modules implementing the PHY layer receive a signalfrom the communication medium 204 and generate an MPDU for the MAClayer. An AFE module 230 operates in conjunction with an Automatic GainControl (AGC) module 232 and a time synchronization module 234 toprovide sampled signal data and timing information to a discrete Fouriertransform (DFT) module 236.

After removing the cyclic prefix, the receiver 206 feeds the sampleddiscrete-time symbols into DFT module 236 to extract the sequence of Ncomplex numbers representing the encoded data values (by performing anN-point DFT). Demodulator/Decoder module 238 maps the complex numbersonto the corresponding bit sequences and performs the appropriatedecoding of the bits (including deinterleaving, error correction, anddescrambling). The data that was modulated onto carriers that weresubsequently attenuated by the spectral shaping module 400 can berecovered due to the redundancy in the encoding scheme.

The adaptive carrier selection technique can be used in combination withthe amplitude mask technique described in U.S. application Ser. No.11/493,382, incorporated herein by reference. For example, the reservedcarriers can be included in a master tone mask for all symbols andselectively turned on or off, as described herein, using the amplitudemask. The amplitude mask specifies an attenuation factor α for theamplitude A′_(i)=αA_(i) according to the amount by which the power is tobe attenuated (e.g., 2 dB in amplitude for each 1 dB in power). Theamplitude A′_(i) is set below a predetermined amplitude that is normallyused for modulating the information (e.g., according to a predeterminedconstellation) such that the resulting radiated power does not interferewith other devices. The amplitude mask entry may also indicate that acarrier is to be nulled completely with the corresponding amplitude setto zero. The attenuated carriers can still be processed by the receivingstation even if they are transmitted with zero amplitude so that themodulation and encoding scheme is preserved.

Any of the modules of the communication system 200 including modules inthe transmitter 202 or receiver 206 can be implemented in hardware,software, or a combination of hardware and software.

Many other implementations of the invention other than those describedabove are within the invention, which is defined by the followingclaims.

1. A method for communicating over a network of communication stations,comprising: monitoring for reception of an electromagnetic wave that hasa power above a threshold and a frequency in a predetermined frequencyinterval; and selecting carrier frequencies for modulating informationonto signals transmitted over the network, based at least in part on ademand by one or more stations in the network for capacity forcommunicating over the network, and ensuring that carrier frequencies inthe frequency interval are not selected for modulating information ontosignals transmitted over the network if an electromagnetic wave having apower above the threshold and a frequency in the frequency interval hasbeen received in a predetermined time interval.
 2. The method of claim1, wherein the demand for capacity by a station comprises a request fortransmission of data from the station at a rate that exceeds anavailable capacity over a communication medium shared by multiplestations in the network.
 3. The method of claim 1, further comprisingensuring that carrier frequencies in the frequency interval that are notselected for modulating information onto signals transmitted over thenetwork are not used to transmit more than a tenth of the powertransmitted using selected carrier frequencies.
 4. The method of claim1, wherein the monitoring comprises monitoring for reception of one ormore qualified electromagnetic waves that have a power above a thresholdand a frequency in one of multiple predetermined frequency intervals inthe predetermined time interval.
 5. The method of claim 4, wherein agiven carrier frequency in a subset of carrier frequencies is selectedfor modulating information onto signals transmitted over the network ifno qualified electromagnetic wave is received in a correspondingfrequency interval that includes the given carrier frequency and if atleast one station demands capacity for communicating over the network.6. The method of claim 5, further comprising terminating use of thegiven carrier frequency for modulating information onto signalstransmitted over the network if a qualified electromagnetic wave isreceived in a corresponding frequency interval that includes the givencarrier frequency or if remaining demand for capacity can be satisfiedwithout using the given carrier frequency.
 7. The method of claim 5,further comprising ensuring that the carrier frequencies in the subsetthat are not selected for modulating information onto signalstransmitted over the network are not used to transmit more than a tenthof the power transmitted using selected carrier frequencies.
 8. Themethod of claim 4, wherein a given carrier frequency in a subset ofcarrier frequencies is selected for modulating information onto signalstransmitted over the network if no qualified electromagnetic wave isreceived in a corresponding frequency interval that includes the givencarrier frequency.
 9. The method of claim 8, further comprisingterminating use of the given carrier frequency for modulatinginformation onto signals transmitted over the network if a qualifiedelectromagnetic wave is received in a corresponding frequency intervalthat includes the given carrier frequency.
 10. The method of claim 8,wherein the selected carrier frequencies in the subset are used formodulating a first category of information onto signals transmitted overthe network and are not used for modulating a second category ofinformation onto signals transmitted over the network.
 11. The method ofclaim 10, wherein the first category comprises information that isunicast to one other station.
 12. The method of claim 10, wherein thefirst category comprises information that is modulated onto a signalusing a modulation format that is adapted to a communication channelbetween two stations.
 13. The method of claim 10, wherein the secondcategory comprises information that is broadcast to all stations in thenetwork.
 14. The method of claim 10, wherein the second categorycomprises preamble or overhead information modulated onto signalstransmitted over the network.
 15. The method of claim 8, wherein theselected carrier frequencies in the subset are used to modulateinformation according to a predetermined encoding that does not dependon which carrier frequencies in the subset are selected for modulatinginformation.
 16. The method of claim 15, wherein the predeterminedencoding includes interleaving data in a manner that does not depend onwhich carrier frequencies in the subset are selected for modulatinginformation.
 17. The method of claim 8, wherein at least some carrierfrequencies not in the subset of carrier frequencies are used tomodulate information according to a predetermined encoding that does notdepend on which carrier frequencies in the subset are selected formodulating information.
 18. The method of claim 17, wherein thepredetermined encoding includes interleaving data in a manner that doesnot depend on which carrier frequencies in the subset are selected formodulating information.
 19. The method of claim 17, wherein the carrierfrequencies not in the subset are used for modulating information ontosignals transmitted over the network regardless of whether any qualifiedelectromagnetic waves is received in any frequency intervals.
 20. Themethod of claim 17, wherein the selected carrier frequencies in thesubset are used for modulating redundant information that corresponds tothe information modulated on the carrier frequencies not in the subset.21. The method of claim 20, wherein the selected carrier frequencies inthe subset are used for modulating the redundant information if theinformation modulated on the carrier frequencies not in the subset isbeing broadcast to multiple stations in the network.
 22. The method ofclaim 8, wherein the selected carrier frequencies in the subset are usedfor modulating information onto signals sent by a subset of thestations.
 23. The method of claim 22, wherein the subset of the stationscomprises stations that transmit the largest amount of data over thenetwork.
 24. The method of claim 22, wherein the subset of the stationscomprises stations that transmit data over the network for the largestamount of time.
 25. The method of claim 22, wherein the subset of thestations comprises stations that use the largest amount of a capacityover a communication medium shared by multiple stations in the network.26. The method of claim 1, further comprising transmitting from a firststation in the network to at least one other station in the networkinformation characterizing the demand for capacity.
 27. The method ofclaim 26, wherein the information characterizing the demand for capacitycomprises information characterizing a demand for capacity by the firststation.
 28. The method of claim 1, further comprising transmitting fromfirst a station in the network to at least one other station in thenetwork information indicating whether an electromagnetic wave having apower above the threshold and a frequency in the frequency interval hasbeen received.
 29. The method of claim 28, wherein the informationindicates whether the electromagnetic wave has been received at thefirst station.
 30. The method of claim 28, wherein each station in thenetwork monitors for reception of an electromagnetic wave that has apower above a threshold and a frequency in a predetermined frequencyinterval.
 31. The method of claim 28, wherein selecting the carrierfrequencies is performed in a distributed process by multiple of thestations in the network.
 32. The method of claim 1, wherein a firststation selects the carrier frequencies and broadcasts informationspecifying the selected carrier frequencies to other stations in thenetwork.
 33. The method of claim 32, wherein the first station selectsthe carrier frequencies based on whether the electromagnetic wave hasbeen received at the first station.
 34. The method of claim 32, whereinthe first station selects the carrier frequencies based on whether theelectromagnetic wave has been received at a station other than the firststation.
 35. The method of claim 1, wherein transmitting signals overthe network comprises transmitting signals over a wired communicationmedium.
 36. The method of claim 35, wherein the wired communicationmedium comprises a powerline network.
 37. The method of claim 1, whereinthe electromagnetic wave comprises a radio wave.
 38. The method of claim37, wherein the predetermined frequency interval comprises an amateurradio band.
 39. The method of claim 37, wherein the radio wave has afrequency within a band regulated by a government agency.
 40. Anapparatus, comprising: a monitoring module including circuitry formonitoring for reception of an electromagnetic wave that has a powerabove a threshold and a frequency in a predetermined frequency interval;a modulation module including circuitry for modulating information ontosignals transmitted over a network; and a carrier selection moduleincluding circuitry to select carrier frequencies for use by themodulation module, based at least in part on a demand by one or morestations in the network for capacity for communicating over the network,wherein carrier frequencies in the frequency interval are not selectedfor modulating information onto signals transmitted over the network ifan electromagnetic wave having a power above the threshold and afrequency in the frequency interval has been received in a predeterminedtime interval.